1. Field of the Invention
The present invention relates generally to the field of mechanical processing of materials such as isostatically pressing materials and more specifically, to an apparatus and method for synthesizing and densifying parts by exploiting the anthropomorphic qualities of shape memory alloys serving the dual function of reaction vessel for self-sustaining combustion synthesis and pressing die for densifying the reaction product of such synthesis.
2. Prior Art
The self-sustaining combustion reaction (sometimes called self-propagating high temperature synthesis, SHS, or gasless combustion synthesis) has been studied quite intensively in the Soviet Union since the later 1950's and in the United States only recently. This reaction route for synthesizing a number of refractory compounds such as oxides, carbides, nitrides, cermets or intermetallics exploits an unique high temperature, highly exothermic reaction phenomenon. It has been demonstrated that when two or more solid reactants (in the form of compacted powder mixture) is intimate contact and when the high temperature, highly exothermic reaction is triggered by an external energy source, the exothermic reaction heat released during the reaction is capable of heating the unreacted reactants to a high temperature and sustaining a combustion wave for the continuation of the reaction. The high temperature generated by the exothermic reaction also accelerates the consolidation of the reaction products. Nevertheless, it is very difficult, if not impossible, to obtain a densified product form without applying external pressure during and/or after the reaction occurs. A detailed description of this reaction can be found in the book titled "Gasless Combustion Synthesis of Refractory Compounds" which is authored by W. L. Frankhouser et al and published by Noyes Publications, New Jersey, 1985. The February, 1986 edition of Advanced Materials and Processes also deals with this phenomenon in an article entitled "Powders That `Explode` Into Materials" by Laurel M. Sheppard at pages 25-32.
The material that has come to be called shape memory alloys has been known for some time. By way of example, in an article entitled "Shape Memory Alloys" written by L. MacDonald Schetky and appearing in the Scientific American, November 1979 issue beginning at page 74, the author thoroughly explains the mechanical properties of these alloys and indicates that they first came to worldwide attention in 1962. As the author in that article explains, the phenomenon of shape memory alloys is dependent upon the characteristic of certain materials to possess a martensite crystal structure deriving from a parent crystal phase when treated by a certain combination of stress and temperature processing. This martensitic crystal phase can then be transformed back into its parent crystal phase thermoelastically by elevating the temperature of the material. In addition, for the "two way" shape memory alloys, the phase transformation as well as the shape can be reversed by then lowering the temperature of the material. As a result of these phase transformations the material can be induced to change its shape in a carefully controlled and reversible manner by simply controlling the temperature of the material to induce the aforementioned phase transformations. The article by Schetky illustrates a particular exploitation of this phenomenon for automatically deploying an antenna for a spacecraft. An updated description of the "Shape Memory Effect Alloys" can be found in the Encyclopedia of Materials Science and Engineering, Vol. 6, pages 4365-4374, published by M.I.T. Press, 1986. However, it is believed that the use of this phenomenon in a press or die for applying reasonably high mechanical pressure forces for pseudo-isostatically pressing one or more reaction parts of self-sustaining combustion reaction is a unique application of this shape memory phenomenon which has heretofore not been disclosed in the art. The most relevant prior art known to the applicants in this regard will now be disclosed.
U.S. Pat. No. 3,353,954 to Williams may be considered the most relevant prior art in that it discloses a self-sustaining combustion reaction process while subjecting the reactants to pressure. However, the pressure is not derived from the synergistic effect of the exothermic reaction on a shape memory alloy press as is taught herein.
U.S. Pat. No. 3,558,369 to Wang et al is directed to a method of effecting a reversion back to an original configuration by means of a martensitic transition of a metal alloy. The alloy is subjected when in an original configuration to deformation at a temperature below a critical temperature to change the shape and then heat the alloy above the critical temperature to effect a reversion.
U.S. Pat. No. 3,579,805 to Kast is directed to a method of forming interference fits by heat treatment. In this patent, members are provided which are formed of a precipitation hardenable alloy composition which undergoes irreversible dimensional changes upon heat treatment. The precipitation hardenable alloy is solution annealed at a temperature higher then the solutioning temperature of the alloy. The members are interfit to form an assembly with a clearance between the members which is less than the irreversible dimensional change and the assembly is subjected to a precipitation hardening heat treatment which causes the irreversible dimensional change.
U.S. Pat. No. 3,832,763 to Schober is directed to a method of drop-forging sintered workpieces. This disclosure provided essentially for a deviation of the shape of the drop forging die wherein a solid cylindrical pre-stressed body is placed into the cavity of a forging die. The cavity of the forging die has a lateral dimension normal to the forgoing direction wherein this lateral dimension is larger than the diameter of the body. There is no reference to a shape memory alloy die.
U.S. Pat. No. 4,019,925 to Nenno et al is directed to metal articles having a property of repeatedly reversible shape memory effect and the process for preparing such which includes deformation stress to a martensitic alloy.
U.S. Pat. No. 4,036,669 to Brook et al is directed to a mechanical pre-conditioning method and to a mechanical composition capable of undergoing a reversible transformation between the austenitic and martensitic states. The article is deformed from an original configuration into a second predetermined shape from which heat recovery towards the original configuration is desired. Constraining forces are applied to the article and the article is held in the deformed configuration at a predetermined high temperature at which the formation of the martensite in the metallic composition is induced thermally in the absence of applied stress for a predetermined time. This causes a portion of the deformation to be retained as heat recoverable strain.
U.S. Pat. No. 4,149,911 to Clabburn is directed to a memory metal article and a method of making the heat recoverable memory mmetal member. A stress is applied to the member in a deformed dimensionally heat-unstable state and such temporarily increases the temperature at which formation of austenite begins. This is accomplished while maintaining an applied stress. The article is stored at a temperature less than the higher temperature and the memory metal member then remains in a martensitic state.
U.S. Pat. No. 4,198,081 to Harrison et al is directed to a heat recoverable metallic coupling. A coupling is installed on pipes to be connected and the joint is heated to greater than the transition temperature of the material of the coupling. The coupling is heat recoverable and recovers or shrinks to the heat-stable configuration until it engages the objects and is restrained from further recovery. This creates a tight fit on the object as long as the joint is maintained above the transition temperature. The restraining action of the objects on the coupling introduces non-thermally recoverable plastic deformation stresses into the material of the coupling and when the coupling is cooled to the transition temperature, the stresses are released in spontaneous expansion and the coupling may be removed from the objects. U.S. Pat. No. 4,283,233 to Goldstein et al is directed to a method of modifying the transition temperature range of a nickel-titanium based shape memory alloys. This is done by the selection of the final annealing temperature. The alloy however is formed into a predetermined permanent shape.
U.S. Pat. No. 4,533,411 to Melton is directed to a method of processing nickel/titanium base shape-memory alloys and structures. The reference teaches cold working an alloy formed of a nickel-titanium based shape memory alloy and provides the alloy in some desired shape while maintaining dislocation-free cells which are obtained in the annealing step. The alloy is deformed in the martensitic state whereby when the alloy is recovered by heating the alloy to the austenitic state and then again cooled to the martensitic state, the alloy will retain the predetermined contour.
Other prior art patents which may be deemed to be relevant to the present invention comprise the following: U.S. Pat. Nos. 3,285,470, Frei et al; 3,622,941, Wetmore; 3,652,969, Willson et al; 3,726,643, Merzhanov; 3,783,037, Brook et al; 4,035,007, Harrison et al; 4,045,644, Shafer et al; 4,067,752, Brook et al; 4,113,475, Smith; 4,161,512, Merzhanov; 4,310,354, Fountain et al; 4,365,996, Melton et al; 4,412,872, Albrecht et al; 4,483,174, Goodfellow; 4,489,964, Kipp et al; 4,518,444, Albrecht et al; 4,554,027, Tautzenberger et al.